This invention relates to improvements in electrical fuses and more particularly to time delay fuses of the ferrule type.
Time delay fuses of the ferrule type have heretofore commonly comprised a cylindrical tube of insulating materials having caps which extend generally axially from the opposite ends of the tubes. Electrically connected between the caps within the tube is a series of longitudinally spaced short circuit elements and a thermal metallic mass of high heat conductivity constituting a structure that is heatable in a current overload connection. Examples of such fuses are found in U.S. Pat. Nos. 4,417,224; 4,414,526; and 4,562,420.
The present invention is concerned with a ferrule type fuse with three separate chambers spaced axially along the tube and forming the short circuit elements on each end of the overload chamber. Three chamber fuses have advantages in fuses of higher current and voltage ratings where more mass and break distance is required. In addition, the use of short circuit elements on each end of the overload section has the added advantage of better heat balance within the fuse and isolates the overload section from the ferrules which can develop hot spots due to poor contact between the ferrule and its mounting fuse clip. Thus, one feature of this invention is that it is adaptable to a wide range of voltage and current ratings for example current rating up to about 60 amps.
Another feature of the present invention is that it has a sub-assembly means which can be pre-assembled comprising a plunger and plunger guide with short circuit elements attached to the opposite ends of said plunger and plunger guide.
Another feature of this invention is that the use of fiber isolating washers are no longer needed to maintain the separate chambers. The overload chamber is closed off on one end by a close fit between the plunger and the tube. The other end of the overload chamber is closed off by a washer ring which rests on a support formed by stakes in the tube wall of the fuse.
Accordingly, it is an object of this invention to design a fuse of the ferrule type which can be assembled by a method that utilizes automatic machinery.
It is a more specific object of this invention to design a three compartment fuse of the ferrule type with a sub-assembly which comprises a plunger and plunger guide, and two short circuit elements, a compression spring and a ring which is designed so that it can be automatically assembled.